1. Field of the Invention
This invention relates to a III-V Group compound semiconductor light-emitting element and, more particularly, to a light-emitting element in which an improved electrode with a multi-layer structure is mounted on the surface of a p-type semiconductor layer involved in forming the p-n junction of the light-emitting element.
2. Description of the Prior Art
Compound semiconductors are widely used for the production of light-emitting elements. Particularly, a light-emitting diode utilizing gallium phosphide (GaP), which is a typical III-V Group compound semiconductor emits light of desired colors ranging from red to green depending on the kind of impurity added thereto. It is, therefore, attracting increased attention in this field.
In general, a GaP light-emitting diode is known which comprises an n-type GaP substrate, an n-type GaP layer formed on the substrate, and a p-type GaP layer formed on the n-type GaP layer, a p-n junction being formed between the n-type and p-type GaP layers. Electrodes are mounted on the upper surface of the p-type GaP layer and on the lower surface of the substrate, respectively. The diode of this structure is mounted on a header by using a conductive paste such that the electrode mounted on the substrate is fixed to the conductive paste. Also, the electrode mounted on the p-type GaP layer is connected to the terminal of the header via a lead wire.
It is necessary for the electrode mounted on the p-type layer to meet various requirements including the following:
(a) The electrode should be brought into ohmic contact with the p-type layer and the contact resistance should be small.
(b) The lead wire should be easily bondable to the electrode.
(c) The electrode should not decrease the light-emitting efficiency of the diode.
(d) The electrode should withstand a strong acid used for etching the p-n junction region damaged by dicing.
(e) The electrode should permit fine processing.
However, an electrode meeting all of these requirements has not yet been developed. Specifically, an electrode with a two-layer structure consisting of a first metal layer formed of a gold alloy containing 1 to 2% by weight of beryllium or zinc, said first metal layer being formed on the p-type GaP layer, and a second metal layer consisting of gold and formed on the first metal layer is known to meet requirement (a) mentioned above. The prior art electrode mentioned certainly meets requirement (d) as well and requirement (e) to some extent, but fails to meet requirements (b) and (c).
Concerning requirements (b) and (c), it should be noted that the two metal layers formed on the p-type layer are selectively removed so as to provide a pattern of electrodes and, then, subjected to heat treatment to achieve an ohmic contact between the electrode and the p-type layer. During the heat treatment, gallium and phosphorus atoms, particularly gallium atoms contained in the p-type layer, are allowed to migrate through the first metal layer (alloy layer) so as to be deposited on the surface of the second metal layer (gold layer). In addition, beryllium or zinc atoms contained in the first metal layer also migrate so as to be deposited on the gold layer surface. As a result, the atoms deposited on the gold layer surface form a film of complex oxides; e.g., Ga-P-Be-O or Ga-P-Zn-O, which is hard to etch away. The oxide film impairs the bonding property of the electrode to the lead wire so much that the bonding operation must be repeated four or five times to achieve the desired bonding.
Further, the gallium atom migration mentioned above results in changes in the GaP crystal structure, leading to decreased light-emitting efficiency of the diode. The difficulty can not be overcome even if the upper gold layer of the electrode is thickened because the gallium atom diffusion coefficient is larger in gold (increase in gold layer thickness simply results in an increase in the gallium atom storage volume).
To overcome the above-noted drawbacks of the prior art, it has been proposed to form an intermediate metal layer between the first and second metal layers. For example, it has been proposed to provide an intermediate layer formed of a metal having a high melting point such as molybdenum in Japanese Patent Application Disclosure No. 53-110460 disclosed to the public on Sept. 27, 1978. It has also been proposed to provide an intermediate layer formed of Ta, W or Nb in Japanese Patent Application Disclosure No. 54-11689 disclosed to the public on Jan. 27, 1979. However, even these techniques leave room for further improvement.